To Answer your Question, when SEP version 12.1 clients are deployed with Packages from SEPM, these packages are created with Latest definitions in it.

So as soon as SEP clients gets installed, the clients are installed with latest definitions.

Now, incase, these clients are Mobile machines, and then connect to the SEPM for definitions however, say incase, they are unable to fetch the entire downloads, then the next time it downloads it would download the same definitions again.

i.e; It would fetch those days definitions which have not been received by Clients.

Here are few Article for you:

Symantec Endpoint Protection 12.1: Installing the Manager for the first time and deploying clients

Yeah we know how to make new installation package and that there's newest definitions on it.

But if client deploy take 2-3 month to get all clients installed - all over the world (multiple sccm sites) its more easiest to copy newest definitions to installation package - or then we have to make installation media all the time and then copy it to sccm sites..

(we're moving SCS to SEP using diffent management servers)

And yes, it download newest definitions when installing sep to computer, but it make (for customer view) such a big load on network that this is why I ask this one.